This invention relates to sphere-like magnetite particles showing an apparent density of from 0.40 to 1.00 g/cm.sup.3 and containing from 0.1 to 5.0 atomic % of Si to Fe and excellent in thermo-stability and dispersibility, and relates to a process for producing the sphere-like magnetite particles.
The sphere-like magnetic particles powder is mainly used as the black pigment for paints and the magnetic toner material for electrostatic reproduction.
Heretofore, magnetite particles powder has generally been used broadly as the black pigment and, in view of the improvement in the working efficiency for the current demand on energy-saving and the enhancement in the physical property of the coated films, improvement in the dispersibility of the magnetite particles powder into vehicles has been more and more demanded in the production of paints.
In the production of the paint, the dispersibility of pigment into the vehicles greatly influences the working efficiency in the paint production step and also constructs an extremely important factor for determining the various physical properties of coated films.
This fact is apparent, for example, from the descriptions on page 8 of Shikizai Kyokai-shi (Journal of the Japan Society of Color Material) vol. 49, No. 1 (1976) are follows. "... Briefly speaking, it may not be exaggeration to say that various properties required for the coated films are mostly determined by the dispersibility of a pigment used therein if the same pigment is used. If the dispersibility of a pigment in the coated film is better, it is clear from the theory that its color becomes clear, and those fundamental properties of the pigment such as the tinting strength and the hiding power can also be improved. Further, the gloss, clearness, mechanical property, gas barrier property, etc. of the coated films can also be improved and as the results the durability of the coated film can be improved. It will thus be understood that the dispersibility of the pigment in the coated films is an extremely important factor that determines the various physical properties of the painted films."
On the other hand, with the remarkable wide spread use of electrostatic copying machines in recent years, extensive research and development have been conducted for the magnetic toners as the developer, and the improvement in the performance of the magnetic toners have been demanded.
For instance, Japanese Patent Application Laid-Open No. 122129/1979 describes as follows. "... Magnetic toners are produced by incorporating a considerable amount of the magnetic particles into the toner binder. However, since the dispersibility of the magnetic particles into the toner binder resin is poor, it is difficult to produce uniform toners with no scattering in the quality upon production of magnetic toners. Further, in the case of the insulative toners, this causes the reduction of electric resistance of the toners". It is also described in Japanese Patent Publication No. 21656/1978 as follows. "... It is possible to obtain an adequate magnetization required for visualizing the static images by the uniform distribution of iron oxide over the entire developer particles."
The magnetic toners are generally manufactured by melting and kneading magnetic particles such as magnetite particles and a resin under heating, cooling to solidify the mixture, pulverizing the cooled material and further passing the pulverized material in an atomized state through a hot air stream to perform spheroidizing treatment. Further, upon development, heat-fixing or pressure-fixing is carried out for fixing the magnetic toners.
Accordingly, the magnetite particles as the material for use in magnetic toners are exposed to a high temperature during production and development of the magnetic toners as described above, and as the results, the black magnetite particles are turned into brown maghemite particles at a temperature from about 200.degree. C. to about 300.degree. C., further into reddish brown hematite particles at a temperature of about 500.degree. C. and, at the same time, the particles lose their magnetic property. Therefore, magnetite particles with excellent thermo-stability have been demanded.
It has been known that upon producing magnetic particles by blowing an oxygen-containing gas through an aqueous reaction suspension containing ferrous hydroxide which has been obtained by reacting an aqueous ferrous solution and an alkali, the shape of the produced magnetite particles varies depending on the pH value in the aqueous reaction suspension.
This fact is apparent from the descriptions in "The Summary of Lecture in 1971's Autumn Meeting of Journal of the Japan Society of Powder and Powder Metallurgy", p.112, lines 14-19 as follows.
"An aqueous sodium hydroxide solution (40-44 g/0.3 l) was added to an aqueous suspension of ferrous sulfate (139 g/ 0.7 l) while blowing air under agitation and the reaction mixture was warmed to 50.degree. C. for 5 hours to obtain fine particles. The pH value of the suspension was changed for varying the shape of the particles. The pH value was adjusted by controlling the amount of sodium hydroxide to obtain pseudo-hexahedral particles in the acidic region (NaOH: 40-41 g/0.3 l), octahedron particles in the alkaline region (NaOH: more than 43 g/0.3 l) and polyhedral sphere-like particles in the neutral region (NaOH: 42 g/0.3 l)", as well as from the descriptions in the claim of Japanese Patent Publication No. 668/1969 as follows. "... producing precipitates comprising black ferromagnetic particles (magnetite particles) of granular or cubical (hexahedral) shape .... by maintaining the aqueous suspension containing colloidal Fe(OH).sub.2 at a pH not less than 10 to a temperature in the range of 45 to 70.degree. C., and carrying out oxidation reaction under the state where precipitated particles present in the liquid are put to sufficient movement by the agitation".
Although it has been most keenly demanded at present to provide magnetite particles excellent in the dispersibility and the thermo-stability, the particles obtained by the foregoing known methods for producing the magnetic particles can not yet be considered excellent in the dispersibility and the thermo-stability.
For instance, Japanese Patent Application Laid-Open No. 35900/1974 discloses a process for producing sphere like magnetic particles from the entire amount of Fe.sup.+2 in the aqueous suspension of ferrous salt.
Namely, the process as disclosed in Japanese Patent Application Laid-Open No. 35900/1974 comprises a first step of adding alkali metal carbonate to an aqueous mixed solution of a water soluble divalent metal salt (Fe.sup.+2 is entirely or partially substituted with divalent metal such as Co.sup.+2) and a ferrous salt, by an amount less than the equivalent to acid radicals contained in an aqueous mixed solution and carrying out oxidation reaction at a temperature lower than the boiling point thereby forming the matrix of ferromagnetic particles, and a second step of adding an alkali metal hydroxide in an amount sufficient to precipitate all of the unreacted metal ions remaining in the solution on the matrix of the ferromagnetic particles thereby forming ferromagnetic particles (in which MOFe.sub.2 O.sub.3, M:Fe.sup.+2 is partially or entirely substituted with divalent metal such as Co.sup.+2).
However, magnetite particles obtained by the above-mentioned process are still not satisfactory in view of the sphericalness of magnetite particles as shown in the Comparative Example 3 described later, in which the resultant particles are coagulated with each other and have only a low apparent density. This may be attributable to the fact that the magnetite particles obtained by the process as disclosed in Japanese Patent Application Laid-Open No. 35900/1974 are formed by hydrolysis reaction of the iron carbonate obtained from a ferrous sulfate and an alkali metal carbonate in the first step and accordingly, magnetite particles are precipitated rapidly and as the result, it is difficult to sufficiently control the shape of the particles.
Furthermore, in the case of forming the sphere like magnetite particles in the aqueous solution of the neutral pH range, since it is difficult to entirely convert Fe.sup.2+ present in the aqueous suspension of ferrous salt into magnetite particles and unreacted Fe.sup.2+ partially remains therein, the production yield is lowered and, in addition, unreacted Fe.sup.2+ causes public pollution due to the discharge water, and as the results, an adequate countermeasure therefor is required.
However, in order to entirely convert Fe.sup.2+ present in the aqueous suspension of ferrous salt into the magnetite particles for improving the yield, the aqueous suspension of ferrous salt has to be reacted with an alkali in an amount more than the equivalent to the ferrous salt in the aqueous suspension. In this case, an alkali reaction solution at a pH value of higher than about 11 is formed, in which the resultant magnetite particles are hexahedral or octahedral particles having a low apparent density, an increased oil absorption and insufficient dispersibility and tinting strength.
The present inventors, taking notice of the shape of the magnetite particles, considered that magnetic particles with excellent dispersibility and high coloring value are those sphere-like particles showing a high apparent density and a low oil absorption, that the improvement in the sphericalness of the magnetite particles can decrease the area of contact between the particles to reduce the coagulation between the particles and increase the apparent density and as the result, the magnetite particles with excellent dispersibility are provided Then, the inventors have made various studies on the process for producing sphere-like magnetite particles showing an improved sphericalness from the entire content of Fe.sup.2+ present in the aqueous suspension of ferrous salt and have attained the present invention.